Plastic caps aren't just made of plastic; they are also made of vinyl, silicone and rubber. Plastic is a wide and varied medium, and each type of plastic cap has its own benefits.
Flexible, silicone caps are extremely tolerant to heat (up to 600 degrees Fahrenheit). As such, silicone caps make excellent high-temperature masks for painting, powder coating, anodizing, E-Coat, and electroplating.
Resistant to heat up to 475 degrees, rubber caps have similar properties to silicone caps and are also used in anodizing and painting. Because they are flexible and provide a tight seal, rubber caps are very useful as noise inhibitors.
Vinyl caps, while less resistant to heat, are also less expensive and come in a wide range of bright, cheerful colors. Vinyl caps are great for protecting products where high-temperature is not a factor, such as threaded implements during shipping. Vinyl caps provide an airtight seal while insulating and cushioning.